Labile Pd-sulphur and Pt-sulphur bonds in organometallic palladium and platinum complexes [(COD)M(alkyl)(S-ligand)]n+—A speciation study

Reaction of various sulphur ligands L (SEt−, SPh−, SC6F4H-4−, SEt2, StBu2, SnBu2, DMSO, DPSO) with the precursors [(COD)M(R)Cl] (COD=1,5-cyclooctadiene, M=Pd or Pt; R=methyl (Me) or benzyl (Bn); DMSO=dimethyl sulfoxide; DPSO=diphenyl sulfoxide) allowed isolation and characterisation of mononuclear neutral (n=0) or cationic (n=1) complexes [(COD)Pt(R)(L)]n+. Reaction of l-cysteine (HCys) with [(COD)Pt(Me)Cl] under similar conditions gave the binuclear cationic complex in [{(COD)Pt(Me)}2(μ-Cys)]Cl. Detailed NMR spectroscopy and single crystal X-ray diffraction in the case of [(COD)Pt(Me)(SEt2)][SbF6] and [(COD)Pt(Me)(DMSO)][SbF6] reveal markedly labilised Pt–S bonds as a consequence of the highly covalent Pt–C bonds of the R coligands in these organometallic species. Cationic charge (n=1) seems to lower the Pt–S bond strength further. Consequently, most of these complexes are not stable long-term in aqueous DMF (N,N-dimethylformamide) solutions. This made the evaluation of their antiproliferative properties towards HT-29 colon carcinoma and MCF-7 breast adenocarcinoma cell lines impossible. Only the two complexes [(COD)Pt(R)(SC6F4H-4)] with R=Me or SC6F4H-4 coligands could be tested with the R=Me complex showing promising activity (in the range of cisplatin), while the R=SC6F4H-4 derivative is largely inactive, as were the phosphane complexes [(dppe)Pt(SC6F4H-4)2] (dppe=1,2-bis(diphenylphosphino)ethane), cis-[(PPh3)2Pt(SC6F4H-4)2] and cis-[(PPh3)2PtCl2] which were tested for comparison. In turn, our findings might pave the way to new Pt anti-cancer drugs with largely reduced unwanted depletion of incorporated drugs and reduced side-effects from binding to S-containing biomolecules.